Dissolution And Precipitation Of Nb In The Heat Affected Zone And Its Effects On Microstructure And Properties

Low-C and high-Nb pipeline steels of X80have being applied and achievedexcellent results in many projects. However, it should understand the existence conditionand strengthing effectiveness of Nb in practical welding process again. It has becomeparticularly important to quantitatively research the precipitation and dissolution of Nband their effects on normal temperature and properties in weld thermal cycles. In thepaper, two Nb-bearing steels are taken as research objects, the regular of Nb dissolutionand precipitation during welding thermal cycles was investigated quantitatively, alsoelucidate the mechanism of Nb in the HAZ definitely.This paper has used a method which determined the content of solube Nb in steel.The method is through dissolving and filtering the sample powders, and then takeadvantages of Inductively Coupled Plasma Atomic Emission Spectroscopic (ICP-AES) toanalysis the content of solube Nb in the liquor. The tested steels were studied by meansof Gleeble-3500thermal mechanical simulator and optical microscopy. The dissolutionregular of Nb in two Nb-bearing steels during welding thermal process was investigated.The results show that the welding coarse grained region remains the excellent strengthand toughness after different welding parameters. The strength of the tested steel islowest at the peak temperature of900℃~1000℃. But the two steels have a poortoughness for the lowest content of dissolution Nb where can produce a large numbermartensite/austenite constituents at the peak temperature of750℃. The content ofsoluble Nb in the HAZ increased with the higher peak temperature which make thestrength and toughness increase significantly and abtain the acicular ferrite.The content of the soluble Nb in the HAZ does not depend on the amount of Nbaddition in matrix after different cooling rates. It depends on the cooling time afterwelding and the heating temperature. The high cooling speed is not conductive to theprecipitation of the soluble Nb. The soluble Nb can reduce the phase transitiontemperature and promote acicular ferrite transformation, also make the CCT curve moveto the lower at right.The contrast of the cooling rate and peak temperature on the microstructure and properties of the welding heat affected zone on the two steels. When the cooling rate ishigher than7℃/s of the high-Nb steel, the performance of the HAZ can meet thestandard requirements. The effect of Nb on the welding HAZ is limited. But in highwelding heat input conditions, the strength of the fine grained region and critical regionin high Nb steel HAZ is reduced significantly. Therefore, for the high Nb X80steel, theresearch should focus on considering the changes in the strength of the welding HAZ inthe future.